PROJECT SUMMARY Our goal is to identify or generate small molecules that safely and effectively lower brain levels of the microtubule- associated protein tau (MAPT) and could be developed into drugs to treat Alzheimer?s disease (AD) and other tauopathies in which tau contributes to neurodegeneration and cognitive decline. Since tau also enables pathogenic effects of other AD-related proteins (e.g., A? and apoE4), reducing tau levels could lessen the effects of multiple drivers of this multifactorial condition and provide a more effective treatment than other monotherapies that have failed in clinical trials. In mouse models, tau lowering in the brain reduces network dysfunction (an early sign of AD) and cognitive decline. Prolonged reduction of cerebral tau levels by up to 75% with antisense oligonucleotides does not elicit overt adverse events in rodents and nonhuman primates. Thus, tau reduction should have limited on-target side effects. Since different forms of tau may contribute to neuropathogenesis, we aim to reduce overall tau levels with small-molecule drugs for the broadest benefit in diverse tauopathies. To discover small molecules that reduce neuronal tau levels, we used high-throughput screening to assess the efficacy of 20,000 structurally diverse compounds in brain neurons. One of these compounds, GL05520, reduced tau protein levels in a dose-dependent manner in rodent and human neurons and reduced neuronal MAPT mRNA levels by up to 75% without affecting transcripts encoding amyloid precursor protein or housekeeping proteins. Preliminary structure-activity relationship (SAR) studies of GL05520 identified three analogs that provide opportunities to improve the physiochemical and pharmacologic properties of the parent compound. Another compound from our primary screen, GL05522, also reduced neuronal tau protein levels but without affecting Mapt mRNA levels, suggesting a different mechanism of action. GL05522 is a potential backup compound if unforeseen liabilities limit the development of GL05520 analogs. Based on the drug-like structures of the tau reducers we identified, we propose a medicinal chemistry plan to improve their potency and pharmacological properties. Optimized compounds will be evaluated in target deconvolution studies to identify their likeliest mechanisms of action, which will facilitate further improvement by an additional round of medicinal chemistry directed at the identified target. Optimized compounds will be tested in standard pharmacokinetic studies of in vivo stability and brain bioavailability. The most promising compounds will be assessed in safety studies. Efficacy studies will test the ability of the lead compounds to reduce brain levels of nonfibrillar and fibrillar tau in rodent models. These studies could pave the way to the development of more effective therapeutics that could transform the treatment of AD and related conditions.